The present invention relates to hydraulic valves and particularly spool valves of the type operated by an electromagnetic solenoid. Valves of this type are employed in control systems for hydraulically actuated devices and in particular where it is desired to provide a modulated control pressure by controlling exhaust or bleed flow from a pressurized source to produce the desired control pressure to the hydraulic circuit for actuating the device to be controlled.
Applications of the aforesaid type are found in controlling the shifting of an automatic power transmission in a motor vehicle. Certain automatic vehicle power transmissions employ a plurality of solenoid operated control valves mounted on the transmission for providing shift control of the hydraulic actuators for the transmission clutches in response to control signals from an electronic controller.
Heretofore solenoid operated valves employed for shift control of vehicle automatic power transmissions have employed spool-type valve members with hydraulic dampening of the spool provided by a chamber at the end of the spool with a restrictive orifice for limiting flow therethrough to thereby provide hydraulic dampening of the spool motion. It is known to provide for filling of the hydraulic dampening chamber from the sump in the vehicle power transmission. However, when the level of fluid in the sump falls below or is drawn away from the orifice in the dampening chamber, the dampening chamber is drained and upon subsequent operation of the valve insufficient hydraulic dampening is provided. Without the hydraulic dampening of the spool motion, it has been found that the valve exhibits instability and erratic operation in response to the energization of the solenoid. This has been found particularly troublesome where the solenoid operated valve is modulated as, for example, by a pulse width modulated signal.
The problem is particularly aggravated by conditions where the vehicle is parked on a grade sufficiently steep to cause the fluid in the transmission sump to drain away from the bleed orifice in the valve resulting in draining the dampening chamber. Thus, it has long been desired to provide a way or means of maintaining fluid in a hydraulic dampening chamber of a spool-type valve, particularly solenoid operated spool valves in a manner which is low in cost and easy to manufacture and assemble in high volume production.
The present invention provides a self-contained fluid reservoir for a dampening chamber in a spool-type valve and which is readily assembled to the valve during manufacture of the valve and which retains fluid in a chamber for continued hydraulic dampening after the valve is withdrawn from the fluid source. The present invention employs a baffled or labyrinth fluid passage for exclusive entry of fluid to the dampening chamber such that upon withdrawal of the valve from the fluid source, sufficient fluid is trapped in the labyrinth to provide hydraulic dampening upon energization of the valve for an initial period until the fluid source is resumed. In the presently preferred practice the invention includes a cap assembled over the spool end of the valve for closing the dampening chamber with a labyrinth entry passage formed in the closed end of the cap. The cap may be snap-locked over the end of the valve dampening chamber for relatively low cost and ease of assembly during manufacturing of the valve.